Extracting oil and natural gas from unconventional resources, such as shale gas formations, through the combination of horizontal drilling and hydraulic fracturing has increased at a rapid pace in recent years. The Marcellus Shale and Utica Shale are sedimentary formations that underlie most of Pennsylvania and West Virginia and extend into parts of Virginia, Maryland, New York and Ohio. These shale formations are two of several important gas reserves in the United States and together they are one of the largest natural gas “plays” in the world. A “play” is the geographic area underlain by a gas or oil containing geologic formation.
Development of these gas plays and other unconventional resources presents significant potential for economic development and energy independence, but also presents the potential for environmental impacts on land, water and air. For example, between 10% and 40% of the water used for hydro-fracturing a gas well typically returns to the surface as flowback, or later as produced water. In addition to fracturing fluids added by drillers, this wastewater picks up other contaminants from deep in the Earth.
In some parts of the United States, gas drilling companies typically dispose of wastewater deep in the ground, by using Class II injection wells. However, the geology in some locations, such as in Pennsylvania, does not necessarily allow for deep injections. Although municipal treatment plants previously accepted this wastewater, certain states, such as Pennsylvania, prevent publicly owned wastewater treatment facilities (POTWs) from accepting water that has flowed back after fracturing without a certain level of pretreatment. This restriction is thought to promote the goal of establishing and maintaining a closed loop process for the recycling and reuse of oil and gas liquid wastes. States other than Pennsylvania also restrict the ability of publicly-owned treatment works to accept oil and gas wastewaters.
Recently, a number of states have passed regulations to treat processed wastewater having specific properties as a non-waste product. For example, General Permit WMGR123 (Pennsylvania Department of Environmental Protection, 2012) identifies specific water quality criteria that, if met, will not require wastewater after it is processed to be treated as waste. The specific criteria of Appendix A of WMGR123 are reproduced below in Table 1.
TABLE IGeneral Permit WMGR123, Appendix A CriteriaPropertyLimitsAluminum0.2mg/LAmmonia2mg/LArsenic10μg/LBarium2mg/LBenzene0.12μg/LBeryllium4μg/LBoron1.6mg/LBromide0.1mg/LButoxyethanol0.7mg/LCadmium0.16μg/LChloride25mg/LCOD15mg/LChromium10μg/LCopper5μg/LEthylene Glycol13μg/LGross Alpha15pCi/LGross Beta1,000pCi/LIron0.3mg/LLead1.3μg/LMagnesium10mg/LManganese0.2mg/LMBAS (Surfactants)0.5mg/LMethanol3.5mg/LMolybdenum0.21mg/LNickel30μg/LNitrite- Nitrate Nitrogen2mg/LOil & GreaseNDpH6.5-8.5SURadium-226 + Radium-2285pCi/LSelenium4.6μg/LSilver1.2μg/LSodium25mg/LStrontium4.2mg/LSulfate25mg/LToluene0.33mg/LTDS500mg/LTSS45mg/LUranium30μg/LZinc65μg/L
Accordingly, it is important that public health and the environment are protected as unconventional resource extraction and production activities become a more prominent component of the oil and gas sector. To this end, regulations governing the management of such wastewater have been evolving at the state level, resulting in increased waste management costs for the petroleum industry. Moreover, strict treatment target requirements specified in each state for unrestricted-use water are particularly challenging to meet. In addition, the federal government has also proposed restrictions on receipt of produced waters by POTWs. Aside from the challenges that may be posed by the regulatory levels for certain contaminants, de-wasting wastewaters from oil and natural gas production pose other challenges, including but not limited to the large fluctuation in daily flow rate of the wastewater; the variation in total dissolved solids (TDS) levels; and variable concentrations of emulsified oil and methanol.
There is therefore a need in the art for methods and systems and for processing oil and gas wastewater with a goal to reuse the processed water, such as for water used in well fracturing while recovering/generating useable byproducts. It would be especially beneficial if such wastewater could be processed to produce both by-products for beneficial use as well as de-wasted water, i.e. unrestricted-use water that is not classified as a residual waste. The production of marketable by-products would reduce the costs of treatment. The production of de-wasted water would allow for less burdensome storage, transportation, and reuse or the potential direct discharge of the water keeping it in the hydrologic cycle.